1. Field of the Invention
The present invention relates to the use of materials which are reticulated solids, such as the photonic band gap (PBG) materials previously claimed in U.S. Pat. No. 5,385,114 (the disclosure of which is incorporated herein by reference, said patent hereinafter referred to as "'114"), as emitter materials in the conversion of thermal or heat energy into optical or light energy, in order to make possible further conversion of the energy from optical or light energy into electrical energy by objects such as well known photovoltaic cells in a more efficient manner than heretofore has been possible.
In particular, one use of these materials is the efficient conversion of thermal or heat energy supplied by flames, sunlight or other broadband electromagnetic sources, or hot gases or fluids into optical or light energy. The light which is produced may be controlled to occur in one or more narrow bands, which permit more efficient conversion from optical or light energy to electricity in photovoltaic cells.
2. Description of the Prior Art
A review of possible applications of PBG materials is presented by Henry O. Everitt in an article entitled, "Applications of Photonic Band Gap Structures", in Optics and Photonics News, volume 3, number 11, pages 20-23, which was published in November, 1992. He includes discussions of resonators, filters, and lasers. Another paper entitled, "Cavity Quantum Electrodynamics at Optical Frequencies", by Morin, et. al., in Optics and Photonics News, volume 3, number 8, pages 8-14, was published in August, 1992. This paper discusses cavities used in quantum physics, and mentions "Yablonovite", a theoretical photonic band gap material. Neither paper discusses the type of thermal to optical conversion contemplated here, which was first alluded to and described in the application which resulted in '114.
A rather old technology for converting thermal energy to optical energy is the Wellsbach mantle, which was patented before 1900. This concept finds application in Coleman lanterns, in which thermal energy from a propane/air flame is converted directly to light. Wellsbach recognized that very fine particles, such as filaments, are readily heated to incandescence, given their small mass. He recognized that materials having small cross sections and long lengths, such as filaments, would tend to be poor thermal conductors. This combination of properties, in a refractory material, yields a structure which produces light with moderately high efficiency from thermal sources. However, significant problems associated with such structures are their lack of ability to withstand mechanical stresses and thermal shocks. We have discovered a class of materials that overcome this difficulty.
At the First National Renewable Energy Laboratory Conference on Thermophotovoltaic Generation of Electricity, held Jul. 24-27, 1994 in Copper Mountain, Colo., there was general agreement that a major unattained technical requirement for efficient thermophotovoltaic systems is stable operation of an emitter at 1800.degree. C. or higher. Most of the presentations at this meeting have been published in the proceedings of the First National Renewable Energy Laboratory Conference on Thermophotovoltaic Generation of Electricity, Copper Mountain, Colo., Jul. 25-27, 1994, published by the American Institute of physics as AIP Conference Proceedings 321 in December 1994. [TPV Conf. Proc.]
Four important reasons were cited for the use of such a high temperature emitter. These reasons are:
1. higher optical power density with higher temperature, leading to higher output power, PA0 2. better matching of emission of radiation at higher temperature to wide bandgap photovoltaic detectors, such as silicon, amorphous silicon, or gallium arsenide, leading to higher power and greater efficiency, PA0 3. smaller losses in electrical output due to bandgap narrowing when wide bandgap materials are used, leading to greater efficiency, and PA0 4. lower resistive losses when higher voltage, lower current (e.g., wide bandgap) photovoltaic detectors are used, leading to higher power and greater efficiency.
It is also apparent that an emitter which selectively emits radiation at or near the wavelength at which the maximum absorption of a photovoltaic detector occurs will tend to increase the efficiency of the overall system, which is a desirable outcome.
We are aware that work on selective emitters that may operate at elevated temperatures is being carried out at NASA Lewis Research Center, Auburn University, and Tecogen. D. Chubb and coworkers at NASA Lewis Research Center are investigating the theory of such emitters, and are measuring optical behavior of systems employing such emitters made from sections of excess rare-earth doped YAG laser crystal rods. NASA Lewis Research Center investigators have told us that they have great difficulty obtaining such materials because of the expense involved, and have no way of having materials made to their express specifications with regard to rare-earth ion content. It is also well known that such laser crystals have marked tendency to crack when heated quickly. We have been informed by prof. Amnon Yogev of the Weitzmann Institute that attempts in his group to heat a ruby laser crystal with concentrated sunlight failed when the crystal cracked under the thermal shock and associated mechanical stresses. The fact that they possess melting points which are typically lower than 2000.degree. C. further makes them incapable of use at 2000.degree. C.
M. F. Rose and coworkers at Auburn University are making emitters using papermaking technology, which involves the preparation of solutions of the ions of interest, followed by the preparation of thin sheets of material which are then fired. From the description they have given at the NREL meeting, the final product includes considerable quantities of silicates and aluminates, as well as rare-earth ions. Such materials are also well known to melt at temperatures below 2000.degree. C. Thin foils such as the paper-like materials being produced are not particularly durable or strong.
Dr. Robert E. Nelson of Tecogen is working in the area of thermophotovoltaic conversion. Nelson has observed the emission of heated lanthanide rare earth oxide fibers tends to occur in a narrow band. Tecogen makes their emitters by a process that involves taking up a solution of a rare-earth nitrate into a yarn, drying the yarn, firing the yarn to produce a filament of rare-earth oxide, and then weaving a structure in a ceramic support using the fiber in a process similar to the making of latch-hook rugs. According to Nelson, the structure that Tecogen has patented is subject to mechanical and thermal stress damage, as the fibers have diameters of micron dimensions, and are not useful at temperatures approaching 2000.degree. C. for extended periods of time. Nelson's structure is not amenable to being heated by a source such as concentrated sunlight because it is composed of both an active material in filamentary form and a support structure of a non-active ceramic, both of which would be heated simultaneously by impinging solar radiation. The support ceramic is neither intended to nor capable of surviving the high temperatures that concentrated solar energy would create.
These prior inventions suffer from a number of limitations and disadvantages which the present invention overcomes. These limitations and disadvantages include low temperature of operation, operation using limited or restricted sources of power, limitations regarding the thermal and mechanical stresses that can be supported, and limitations with regard to emissive power. Our invention provides a solution to each of these problems.
In contradistinction to the materials employed by others, we have discovered how to employ our materials in a manner such that they exhibit very high temperature operation at 2000.degree. C., they can be operated with several different power sources, they exhibit great resistance to both thermal shock and mechanical stress, and they provide very large emissive powers.
In addition, we have discovered other new details of use, which, when taken together, permit our invention to achieve results which the previously disclosed technologies are not capable of achieving. These discoveries will be stated explicitly in the discussion of the invention.
Many embodiments of the present invention can be envisioned. In one instance, the material can be made to operate using flames created by burning hydrocarbon gases such as propane or natural gas in air. In another instance, concentrated sunlight, up to 2000 times as intense as natural sunlight, may be used as a power source. In yet another instance, hot fluid may be used as a power source.
In addition, thermophotovoltaic emitter materials used according to the prescriptions given in the present invention may be used in optical equipment and machines of more advanced design than those manufactured previously.
We will present an example which demonstrates the superiority of the present invention over the prior art in emitter materials for use in thermophotovoltaic applications.